Polyurethane coating materials typically comprise a catalyst, and in this context not only acidic compounds but also, in particular, tertiary amines and/or metallic compounds, such as various tin compounds, for example, more particularly dibutyltin dilaurate and dibutyltin oxide, are employed.
The employment of tin-containing catalysts is to be avoided in coating materials, as elsewhere, because of the toxicity inherent in many tin compounds. The EU Commission's Working Group on Classification and Labelling” have categorized dibutyltin oxide (DBTO) and dibutyltin dilaurate (DBTL) accordingly.
The article available on the Internet at the address www.wernerblank.com and titled “Catalysis of the Isocyanate-Hydroxyl Reaction by Non-Tin Catalysts”, by Werner J. Blank, Z. A. He, and Ed. T. Hessell of the company King Industries Inc., therefore describes alternatives to the typical tin-containing catalysts based on various metal salts and metal complexes, such as zirconium chelates, aluminum chelate, and bismuth carboxylate.
DE 10 2008 061 329 A1 discloses coating materials where the use of metal-containing catalysts is to be avoided as far as possible and which instead as catalyst comprise 1,3 substituted imidazolium salts for the deblocking of blocked polyisocyanates in polyurethane coating materials.
WO04/029121 describes polyurethane compositions which are stabilized in terms of the reactivity of the composition by addition of acids with a pKa range between 2.8 and 4.5, these acids being able to be utilized at the same time as catalyst. Acids used in this context and with a pKa range between 2.8 and 4.5 include, for example, benzoic acid, hydroxybenzoic acid, salicylic acid, phthalic acid, and so on. The compositions preferably comprise no further catalyst, although in addition it is also possible to use the typical known polyurethane catalysts, such as tertiary amines or amidines or organometallic compounds, such as tin compounds more particularly. Where amines are used as catalyst, it is necessary to employ great care in the selection of the type of amine and its amount, since the aminic catalysts are able in part to eliminate the stabilizing action of the organic acids added.
U.S. Pat. No. 5,847,044 describes polyurethane powder coating materials which as catalysts comprise N,N,N′-trisubstituted amidines, more particularly bicyclic amidines.
WO 09/135600 describes polyurethane compositions, more particularly sealants, adhesives, and foams, which comprise as catalyst the reaction product of a metal salt with nitrogen-containing, heterocyclic compounds, more particularly substituted imidazoles.
DE-A-24 34 185 describes a process for preparing amidine-metal complexes and their use as catalysts for the isocyanate polyaddition reaction. These amidine-metal complexes are prepared by reacting an amidine with a 0.5- to 4-fold molar amount of a metal compound, the amidines used comprising not only monocyclic and/or bicyclic compounds, such as imidazoles more particularly, but also acyclic compounds, such as formamidines, acetamidines, benzamidines, and guanidines. Metal compounds used are those of trivalent iron, of divalent nickel, of divalent zinc, of divalent manganese, of divalent tin or of tetravalent tin, with the corresponding carboxylates being employed more particularly.
Lastly, U.S. Pat. No. 7,485,729 B2 and also the equivalent specifications WO06/022899, US 2006/0247341 A1, and US 2009/0011124 A1, describe organometallic compounds and coating materials comprising them. Coating materials described are powder coating materials based on hydroxyl-containing polyacrylates and/or polyesters and on uretdione-containing polyisocyanates, liquid coating materials based on hydroxyl-containing polyacrylates and/or polyesters and on blocked polyisocyanates, and also solventborne coating materials based on epoxy/carboxy or epoxy/anhydride components. The organometallic compounds used as catalyst, besides other metal-amidine complexes, are cyclic or acyclic zinc biscarboxylate-bisamidine complexes, such as Zn(1,1,3,3-tetramethylguanidine)2(2-ethylhexanoate)2, for example.
A problem addressed by the present invention, therefore, was that of providing coating material compositions, more particularly for automotive refinish and for the coating of utility vehicles, that ensure good assembly strength after just a very short time, meaning that they ought to ensure rapid curing even under the conditions of refinish and of the finishing of utility vehicles, in other words ought after curing at 60° C. for 30 minutes already to have undergone curing to an extent such that initial assembly operations or demasking operations can be carried out without damage to the coating. At the same time, however, the coating material compositions ought, at room temperature and after mixing of the binder component with the isocyanate component, to have a good potlife of at least 2 hours. Potlife here means the period of time within which the coating material composition has attained twice its initial viscosity.
Moreover, the coating material compositions ought to lead to coatings exhibiting good through-curing and sufficient ultimate hardness. Furthermore, the coating materials ought not to show any yellowing problems. Thus the catalyst neither must exhibit any intrinsic color and nor must it lead to discoloring at mixing or during curing of the coating material when the catalyst is mixed with the typical components of a coating material. The resultant cured coatings ought, furthermore, to have no tendency toward yellowing after exposure in the test known as the WOM test (WOM=Weather-Ometer Test, determined in accordance with SAE (Society of Automotive Engineers) Standard J2527_04). Yellowing here is determined as the total color difference ΔE using an Xrite MA68 colorimeter, with calculation according to DIN 6174. The yellowing of the coatings here ought not to be worse than the yellowing of the corresponding coatings based on catalysts comprising tin.
Furthermore, the catalyst ought to be able to be added to the coating system from the outset. However, admixing the catalyst to the coating systems from the outset is not to cause any adverse effect on the shelflife of the coating materials. Furthermore, the catalyst ought to be insensitive to hydrolysis, since even in systems in organic solution, the typically high concentration of hydroxyl groups can result in a reduction in catalyst activity over the storage period. Especially in the automotive refinish segment, an extremely long shelflife even at relatively high temperatures is an advantage.
Lastly, the coating material compositions ought to be able to be prepared simply and with very good reproducibility, and ought not to cause any environmental problems during application. More particularly, catalysts containing tin ought to be avoided or at best be entirely dispensible.